I get 17 mpg on long high way cruises (about 75 mph/2800 rpm). Although it is a 24 year old 5 liter engine it seems a bit low for cruising. Or is it normal? Only things I haven't done since I own the car is check the MAF and ECU. Can these cause low fuel economy? What other possible culprits are there: rings, leaking dampers/regulators, carbon build up?

 

Seems normal conpared to mine. I've heard of a few guys getting into the 20s but mine doesn't. Of course it seems I can manage to kill the fuel economy of anything I drive.

 

I get about 18 (before the Supercharger went on) so you are right in there.

 

Has the O2 sensor been replaced?

Knock sensors? Cam angle/hall sensor? Retard will hurt mileage.

Cams at zero?


Autos will get a couple MPG less than manuals.
X-pipe will often drag down MPG - factory cats are best for mileage.



I managed 24.6 mpg (9.6 l/100km) in my '88 5-speed w/X-pipe on a 200+ mile round trip to the beach last month.
Pretty happy with that. Speeds were mostly below 70, though.

 

I get 25 mpg highway with my 86, 22 with the 87. But the 86 has taller gearing.

 

I get 11-12 mpg. Must be that 2.73 rear end, lots of revs.

 

See that makes me wonder. 17 to 18 versus 22 or 24.6 (with x-pipe), that's a huge difference. Can't be all difference auto vs. manual can it?

Ken: O2 sensor has been replaced 5 years/35000 km ago.

 

Another data point, 87 AT I get 20-22 on the freeway, around 65-70mph. Long gears. Tons of variables to account for your slightly low mpg. Biggest might be your driving style. Subtle changes in throttle application result in big mpg changes. I know cause I drive many miles for work and play around with rental cars mpg indicators and watch how it changes with speed or driving style.

 

Yeah but that is in Greg's 12 liter right?

 

I still get 19 to 19.5 on the highway if I don't get into the turbo at all for extended times (no fun in that!). I can drop that down to single digits on our fun runs keeping the turbo on full boil....

 

My '88 with those loping 2.20 gears rewards me with a consistent 20mpg at 70mph on road trips. I really can't complain too much considering what a joy it is to drive.

 

I get 22 combined city/highway in my 81 ( yes I only have half the valves) but my injectors are 24lb'ers.
I rev it and use the fuel enrichment constantly. Could get way better if I could stay at 70mph... but the loud pedal is just too tempting.

 

Wow!! Some of you guys are getting some seriously bad mileage!!

Granted, I have the 4.7, but I get 24.5 on the freeway, and I don't shy away from the accelerator. '84 5 speed, btw.

 

My 86.5 gets about 19 mpg on highway, I think it's good for the age of the car, it's a automatic, but I do have a 1987 928S4 trans in my 1986.5 and now I get about 21 to 23 mpg at 75 mph, tach around at 2000 rpm.

 

I can get 21-22mpg on the highway if I keep it at 65-70.

At 80-85, I get 18-19.

Kept good records of this on my August round trip from NY to San Antone.

The short rearend gearing is stock.

Have a 4.7 Euro S bottom end and heads.

Cams are stock '78 (euroS' left snout broke off)

Induction is stock '78 CIS. ('86 EuroS is in boxes)